This program was initiated to elucidate a newly recognized modality of vaccination and to extend our long-term study of the immune response and clinical sequelae of hepatitis C virus (HCV) infection. One of the advantages of genetic immunization is that the endogenously expressed proteins can be recognized by class I MHC molecules and expressed on the cell surface. The MHC-antigen complex on the cell surface can be recognized by cytotoxic T lymphocytes (CTL), which in turn are activated and attack infected cells. The possibility of inducing an immune response to HCV core protein using DNA immunization provides an attractive alternative to classic vaccination. There are many problematic issues related to vaccine development for hepatitis C. One of the major concerns is the genetic stability of the infectious agent, HCV. There are two hypervariable regions in the putative HCV envelope proteins. Immune escape mutants observed were attributed to mutations in these regions. Experimentally infected chimpanzees and HCV patients were found to have repeated bouts of infection with either homologous or new strains of HCV. This-could also be one of the reasons that more than 80% of the infections become chronic. Directly inducing strong cell-mediated immunity, especially protective cytotoxic T lymphocyte, may not only help in preventing initial HCV infection, but may also produce immune modulation to overcome existing infection. During the past year, we have constructed several different plasmids containing HCV genes and were able to evaluate the induction of antibodies to HCV core proteins in mice. We are in the process of developing assays to measure CTL activity in the mouse model.